Biomechanics of First Ray Hypermobility

In 1887, Dr Joseph Cotterill identified a stiffening of the big toe he termed “hallux rigidus”; a manifestation of first metatarsophalangeal joint osteoarthritis. To date, 133 years after its discovery, we are no further in understanding how it occurs except for a higher-odds ratio among the planus foot type. The majority of clinical and basic-science research of osteoarthritis has concentrated on the hand, hip, and knee. Although large epidemiological studies are best able to identify at-risk populations, new studies need to focus on the unresolved questions related to biomechanical pathways. While many possible etiological factors of hallux rigidus have been dismissed due to a lack of convincing evidence, the role of first ray hypermobility remains enigmatic. However, there is limited understanding of first ray hypermobility and its relationship to foot structure and function. The purpose of this thesis was to provide insight into the biomechanics of first ray hypermobility as a potential etiological factor in hallux rigidus. Four distinct but related investigations were conducted to address current gaps in knowledge: (1) an epidemiology study of population-based trends in hallux rigidus compared to more frequently studied joints; (2) the design and testing of a novel device to standardise measurements of/and quantify first ray hypermobility; (3) investigation of the differences and relationships between foot structure and function caused by first ray hypermobility, and; (4) development, verification, and validation of a finite element model for predictions of cartilage contact mechanics in the hypermobile first ray. Incidence of hallux rigidus was found to be increasing at a rate comparable to the hip and knee. In contrast to other joints, a bimodal age-distribution was found for hallux rigidus, highlighting a subset of younger patients in whom hallux rigidus may be initiated by biomechanical factors other than wear and tear in old age. The novel device for measurements of first ray mobility was found to be substantially more reliable than the standard, clinical exam. Measurements may be performed in partial- and full-weightbearing conditions to facilitate investigation of aberrant foot mechanics resulting from first ray hypermobility. A study of healthy, asymptomatic subjects with planus and rectus foot types established that individuals with first ray hypermobility were predominantly planus in foot type. Subjects who were characterised as hypermobile exhibited increased maximum force beneath the hallux and greater first metatarsophalangeal joint rotational laxity, demonstrating an interaction with translational first ray mobility. Finite element simulations predicted increased first MTP joint stress in the planus foot with first ray hypermobility which, at a magnitude of 6.5 MPa, was within the upper bound of a proposed 5-7 MPa failure limit of cartilage. Taken together, these interlinked studies may elucidate the role of first ray hypermobility in abnormal structure and function of the foot. In the presence of pes planus and hypermobility, an interaction between translational first ray mobility and rotational first metatarsophalangeal joint flexibility may reduce the mechanical advantage from the Windlass mechanism. Concomitant increased force beneath the hallux likely promotes a higher flexion moment arm between the hallucial load and first metatarsophalangeal joint, subjecting the cartilage to potentially harmful tensile and shear stress. Microtrauma to the first metatarsophalangeal joint’s articular soft tissues, after repetitive excessive loading on a daily basis from first ray hypermobility, may initiate degenerative changes. The significance of this research rests on its potential to reveal the interaction between pes planus and first ray hypermobility as an etiological factor in hallux rigidus onset and progression.